relatively little elastin/ smooth muscle in tunica media; tunica adventitia is heaviest up to 65% of blood is in veins at any one time...blood reservoirs? 2 structural adaptations that promote blood return = valves and large internal diameter

Some Terms Blood Flow = need pressure gradient for flow to happen. Can be regulated independently for various tissues and organs Blood Pressure = force per unit area exerted on the wall of a blood vessel by its contained blood (mm Hg) refers to systemic arterial blood pressure in the largest arteries near the heart F = P1 – P2 / R... (Pressure at begin – pressure at end) / Resistance Resistance is the major determinant of blood flow because a change in blood vessel radius increases resistance to the 4th power measures total of frictional forces that impede flow; flow and resistance are inversely related influenced by Blood Viscosity, Vessel Length and Vessel Diameter Blood Viscosity...due to formed elements, plasma proteins...fairly constant Vessel Length...more length = more resistance...does not vary day-to-day Vessel Diameter...can be regulated; fluid not touching walls moves faster Arterioles can regulate their diameters